Field of the Invention
Aspects of the present invention relate to a stable catalyst for liquid redox sulfur recovery processes and circumneutral Fenton-like oxidation processes.
Description of the Prior Art
Complexes comprising ferric ion—Fe(III)—sequestered with suitable chelating reagents have been used as catalysts in liquid redox sulfur recovery (LRSR) processes to control hydrogen sulfide (H2S) emissions and to recover elemental sulfur. These catalysts are also used in Fenton-like oxidation processes carried out in solutions with pH ranging from neutral to alkaline. The chelated iron most often employed in LRSR processes are Fe(III)-Nitrilotriacetate (NTA) and Fe(III)-Ethylenediaminetetraacetic acid (EDTA). A major disadvantage with conventional chelated iron employed in these processes is oxidation of the ligands over time and the consequent eventual loss of catalyst. Although the use of free radical scavengers can prolong the useful life of these catalysts, there is still appreciable catalyst loss over time. A further disadvantage with conventional chelated iron employed in a LRSR process is the relatively high cost of the chelating agents as well as their residual environmental impact.
LRSR processes employing chelated iron can be used to remove H2S from for example natural gas and refinery flue gas (Hua et al., 2002; DeBerry, 1997). In this process, a solution comprising Fe(III)-L (where “L” denotes a conventional ligand such as NTA, EDTA, or similar chelating reagents) at ambient temperature (20-25° C.) absorbs H2S from the gas phase in an absorbing vessel and oxidizes sulfide to elemental sulfur according to the following reactions:H2S(Gas)↔H2S(Aq)↔H++HS−  (1)HS−+2Fe(III)-L→2Fe(II)-L+2H++1/8S8  (2)
The Fe(II)-L formed in the absorber vessel is oxidized by air to Fe(III)-L in another vessel according to the following reaction:Fe(II)-L+1/2O2+H2O→Fe(III)+2OH−  (3)
Therefore, the overall reaction for the oxidation of H2S in these processes is as follows:½O2+H2S→H2O+1/8S8  (4)
Equation (4) shows that under ideal conditions Fe(III)-L acts as a catalyst for the oxidation of H2S by oxygen. However, the actual conditions are less than ideal, and although applied widely, a major disadvantage of the LRSR process is ligand oxidation and eventual loss of iron catalyst. This is because oxidation of Fe(II)-L to Fe(III)-L by air is believed to generate hydroxyl radical (●OH), a powerful oxidizing agent, according to the following reactions:Fe(II)-NTA+O2+H2O→Fe(III)-NTA+H2O2+OH−  (5)Fe(II)-NTA+H2O2→Fe(III)-NTA+OH−+●OH  (6)
The OH-radical generated in the above process reacts with, and degrades, the ligand (e.g., EDTA, NTA) causing iron to hydrolyze and precipitate out as ferric hydroxide (Neumann and Lynn, 1984; Chen et al., 1993).
The oxidation and eventual loss of ligands, and by extension iron-chelate, considerably increases operating costs for the LRSR process and limits its use. Attempts have been made to increase the useful life of Fe(III)-L by: a) synthesis of chelates more resistant to OH-radical attack (Hua et al., 2005); and b) using OH-radical scavengers (Diaz, 1983). Although these strategies were able to reduce the rate of oxidation of ligand and hence its loss, appreciable catalyst loss is still evident over time. Further, the strategies preclude the use of H2O2 to enhance these LRSR processes, which further increases the rate of OH-radical production and hence oxidation of the ligand.
A further disadvantage of employing conventional chelated iron in an LRSR process is that the recovered sulfur is impure and of lesser value than pure sulfur.
Oxidation of sulfide by Fe(III)-L is also a convenient method for generation of Fe(II)-L, which can react with H2O2 (see Equation (6)) in a Fenton-like oxidation process to generate hydroxyl radical; the latter can be used to oxidize organic pollutants in water and soil (Dao and De Laat, 2011 and Cox, U.S. Pat. No. 6,960,330, 2005, Pignatello, U.S. Pat. No. 6,160,194, 2000). Since the iron(III)-L employed in these processes undergoes redox recycling, the oxidation of sulfide to elemental sulfur by iron(III)-L under circumneutral pH provides a potentially fast, convenient, and economical method of generating iron(II) to drive Fenton reactions. However, such use has been limited given the instability of conventional ligands in the presence of hydroxyl radical and the fact that there is presently no economically viable means to employ the Fenton reaction at a pH in the range of about 6-9.